1. Field of the Invention
The present invention relates generally to the field of projection apparatus and projection lamp control and more particularly to a projection lamp control arrangement to accurately maintain the operating point and color temperature of a high intensity projection lamp.
2. Description of the Prior Art
Projection apparatus of the prior art utilizing high intensity projection lamps utilize blower or other ventilation devices to prevent the projection lamp from overheating. Typically, the blower runs at full output capacity independently of the operating characteristics of the projection lamp; namely, voltage and current operating levels, color temperature, etc. The output capacity of the blower depends upon the blower input voltage and thus the blower air flow varies over a rather wide range especially with fluctuations in the input line voltage.
In projection apparatus utilizing metal arc lamps, the cooling of the metal arc projection lamp is critical to obtain constant light output, proper color temperature and a long operating life. The operating point of the metal arc projection lamp depends on manufacturing parameters of the projection lamp and also the air flow or cooling medium of the projection lamp.
During the operating life of the projection lamp, the operating characteristics of the lamp change or age. For example, the electrodes of the projection lamp "burn back" during the operating life. This results in increased operating voltages when the projection lamp is supplied by a constant current projection lamp supply. Accordingly, increased power dissipation results causing a deviation from the desired operating point and a shortened operating life of the projection lamp.
Various arrangements of the prior art have attempted to provide a degree of control over the operation of lamps, discharge tubes and other electronic devices in various environments.
For example, U.S. Pat. No. 930,958 to G. K. Hartung discloses an air flow control arrangement wherein the voltage of a glow lamp is sensed to increased the air flow and maintain a constant operating temperature of the lamp. The air flow is controlled by the positioning of a damper and a central ventilation tube between air inlets and air outlets.
U.S. Pat. No. 2,080,908 to A. Bunger et al, U.S. Pat. No. 2,177,704 to V. J. Francis, U.S. Pat. No. 2,196,022 to E. E. Moyer and U.S. Pat. No. 2,279,941 to C. R. Dunham et al disclose various cooling and blower control arrangements where operation and the state of the cooling arrangement is modified in accordance with operating characteristics of an electronic device. Bunger and Francis are directed to cooling operation to bring about rapid operating cycles of a discharge device with rapid cooling after extinguishing of the discharge device to promote subsequent ignition. Dunham operates the blower when the lamp is in full operation, disables the blower during a heating up process and continues blower operation when the lamp has been extinguished for a rapid cool-down. Various distinct operational modes of the blower are thus determined by the voltage of the lamp. Moyer provides operation of a cooling system to anticipate a rise in temperature of an electrical valve device and includes temperature responsive means to maintain operation of the cooling system as long as the temperature of the electric valve exceeds a predetermined value. Low current of the electrical device is utilized to initiate operation of the cooling system.
U.S. Pat. No. 3,359,454 to W. N. Scheppe discloses a cooling system for a mercury vapor lamp wherein a lamp is not cooled during its warm-up period for fast warm-up and is rapidly cooled when the mercury vapor lamp is off to minimize the restart or recycle time. A two-speed fan blower is utilized to provide the most rapid cooling when the lamp is off and less cooling during normal operation.
U.S. Pat. No. 2,818,530 to C. G. Collar discloses a cooling system for a mercury cathode electron device with the grid current being utilized to control the cooling system. The blower motor of the cooling system remains on as long as the grid current is a predetermined value determined either instantaneously or in a time averaged manner.
U.S. Pat. No. 3,885,194 to B. W. Schumacher discloses an arrangement for controlling the temperature of a heated cathode of a high power electron beam gun to regulate the beam and maintain constant beam current during operation and prior to turn on. The thermionic ignition of a predetermined area of the cathode is measured and the measured signal utilized to control the temperature of the heated cathode.
U.S. Pat. No. 4,101,807 to H. M. Hill discloses a heater element for a low pressure metal or metal halide vapor lamp to control lamp temperature for optimum light output and to decrease the warm-up time of the lamp. An electrical control circuit measures the ratio of the heater voltage to the heater current and compares this ratio against a reference to control the heater current to maintain optimum temperature.
While the above-described arrangements are generally suitable for their intended use, these arrangements do not provide accurate operating point and color temperature control for a high intensity projection lamp in a closed loop by varying the output of a blower in response to sensed deviations from the desired operating point of the projection lamp.